Optical printed circuit board technologies have a wide range of applications which take advantage of their ability to support high bandwidths and other known benefits. For example, it is known to use optical PCBs in aerospace vehicles to convey sensor information to primary processing nodes, or as optical links in large passenger planes to convey sensor and multimedia information across the plane (e.g. video on demand for passengers). They can also be used in automotive applications for the distribution of sensor and media information in cars. They can be used in high performance computing to accommodate huge bandwidth densities around processor nodes, or in a telecommunications setting to accommodate the huge bandwidth densities for embedded optical channels at network nodes. It is also contemplated that optical printed circuit boards may be advantageous in data storage technologies, for example for signal communications in the back planes of disk drive storage enclosures.
Optical printed circuit boards (PCBs) have optical waveguides that are used for the transmission of light signals between components, as well as or instead of conventional copper conductors. Typically, an optical PCB consists of a base or support layer. In areas of the optical PCB where optical waveguides are required, a lower optical cladding layer is provided usually of uniform thickness. On top of this, a layer of optical core material is laid down. The optical core material has a higher refractive index than the cladding layer and will eventually form one or more optical waveguides on the optical PCB.
In a known process used for making optical PCBs, the core layer is laid down in liquid form, e.g. as a curable liquid polymer. A photolithographic mask having a pattern corresponding to the desired shape of the waveguides is arranged over the liquid polymer and the entire resultant structure is then irradiated with electromagnetic radiation of suitable wavelength. Thus, in regions of the mask which are open, the liquid polymer is cured. In other regions, the polymer remains liquid. The mask is removed and the remaining liquid polymer can be washed away with an appropriate solvent known as a “developer” leaving the desired pattern of optical waveguides. Alternatively a dry film optical polymer can be used instead of a liquid polymer.
The remaining core material is typically arranged in patterns of channels which are arranged in some manner so as to be able to couple optical signals between components on the optical PCB when the components are arranged thereon. Last, an upper cladding layer is laid down, so that the channels of core material are completely surrounded by cladding material, and therefore are able to function as optical waveguides. FIG. 1 shows a schematic representation of such a conventional optical PCB and FIG. 2 shows a sectional view of the same optical PCB.
The PCB 2 comprises a base or support layer 4 upon which is arranged a lower cladding layer 6. A plurality of optical waveguides 8 are arranged on the lower cladding layer 6 and have arranged around them an upper cladding layer 10. Thus, the optical waveguides are entirely surrounded by cladding material of the upper cladding layer 10 and the lower cladding layer 6. Typically, the height H of the optical waveguides will be of the order 50 to 70 μm. The width W of each of the optical waveguides is typically of the order 50 to 100 μm. The waveguides can be fabricated to very high accuracy, typically of the order of <1 μm. High accuracy is an important requirement of any optical waveguide structure.
In order to couple optical signals into the waveguides 8, optical components such as transmitters, receivers, 45° mirrors and other optical waveguides, the components must be aligned with respect to the optical waveguide input interfaces or facets 12 with a high alignment tolerance. It is also important to achieve a smooth and normally planar interface at the facets 12 to avoid scattering of light entering or exiting the waveguide 8.
For example, FIG. 3 shows a magnified view of a facet 12 showing a typical roughness profile of a conventionally manufactured optical PCB 2. FIG. 4 shows the effect this roughness produces on light 8 propagating in the waveguide 8 and exiting the facet 12. The roughness of the surface 14 causes scattering of light 24 as the light exits the waveguide 8, causing in turn the attenuation of the optical signal 22. It should be noted that while FIG. 3 shows an output facet 12, the problem of scattering and attenuation of light affects input facets 12 in a similar way. This loss of light is a major problem to be addressed in the manufacture of optical PCBs 2.
Currently, the most effective method of reducing this loss is manual polishing. However, this method is not very effective. Even using this method, the roughness of the facets 12 can cause over 50% of the total optical link loss in the waveguide. Another known technique for reducing attenuation of light due to facet roughness 14 is to use laser ablation of the waveguide facets 12. However, both these methods are unreliable and unrepeatable. In particular, any damage incurred by these methods will be difficult to correct and therefore using these techniques is likely to significantly impact yield in manufacturing optical PCB technology. These methods are also difficult and costly to apply. What is needed is a repeatable, reliable and low cost method of preparing the exposed input and output facets of the waveguide to reduce scattering at the interface.
In instances where non-free space interfaces are used, i.e. where a component couples directly to a waveguide, it is known to apply an index matching oil or gel onto the interface between the waveguide and component to reduce coupling losses. However, this method is unsuitable for an application requiring repeatable engagement and disengagement of the component to the waveguide as the index matching material would flow out during disengagement and attract and hold contaminants (e.g. dust particles), contaminate other components or simply dry up and cease to be effective when next required.